Solid polypropylene is one of the most significant commercial plastics in use today. By use of appropriate conditions and catalysts it can be produced in a sterically regulated form known as isotactic polypropylene. It is difficult, if not impossible, to polymerize propylene to a polymer which has 100% isotactic structure. However, it is possible to produce, with commercially practical catalysts, polymers which have a high proportion of segments that are completely isotactic. A property which is associated with isotacticity of polypropylene is the capacity of a melt thereof to solidify in crystalline form.
Following conventional terminology, reference to "crystalline" or "isotactic" polymers means, unless the context indicates otherwise, solid polymers having a high degree of crystallinity or isotacticity, usually at least 50%, as determined by X-ray analysis or comparable methods. Typically polypropylene having a crystallinity of this order contains less than 10% and usually less than 5% of material which is extractable in boiling heptane or isooctane. Similarly, "crystallizable" polymers are those which have a molecular arrangement that enables them to solidify from a melt in a highly crystalline structure. The general practice in the art is to refer to "crystalline" or "crystallizable" polymer, rather than "partially crystalline" or "partially crystallizable" polymer, even though olefin polymers of 100 percent crystal structure are not known to exist. For example, a crystallinity of 70% is extremely high for isotactic polypropylene. Normally solid, crystalline polypropylene usually has a viscosity average molecular weight of at least about 40,000 and generally between 100,000 and 1,200,000. For convenience the molecular weight is usually expressed in terms of intrinsic viscosity. The intrinsic viscosity of polypropylene, measured in decalin at 150.degree. C. is generally between 1.0 and 6 dl./g. but may be as low as 0.5 or less and as high as 10 or more.
Reference to "polymers" herein includes both homopolymers and copolymers unless the context indicates otherwise.
Crystalline polymers, in their usual form, have some outstandingly good properties and some undesirable ones. For example, desirable properties of highly crystalline polypropylene are high tensile strength and substantial hardness. One disadvantage of the usual forms of highly crystalline polypropylene is a lack of transparency or clarity, which shows up as haze in thin films and as translucency, decreasing to ultimate opacity, in articles of progressively increased thickness. Another disadvantage of the usual forms of highly crystalline polypropylene is a relatively low impact resistance. This handicaps the use of isotactic polypropylene for making vessels or containers which during use may be subject to mechanical shock.
Polypropylene, like many other crystalline polymers, crystallizes from a melt in a form in which the individual crystals are often associated in spheroidal or ellipsoidal bodies known as spherulites. Generally, clarity and some mechanical properties of articles made from polypropylene are better when the spherulites are relatively small.
Methods of "nucleating" polypropylene to improve properties are also well known. U.S. Pat. Nos. 3,207,739; 3,268,499; 3,326,880 and 4,184,026 all disclose the incorporation of various nucleating agents in polypropylene. There are also a number of articles and books describing nucleation in polypropylene. For example, Binsbergen in his article "Heterogeneous Nucleation in the Crystallization of Polyolefins: Part I, Chemical and Physical Nature of Nucleating Agents", Polymer, 11, p. 253-267 (1970), tested a large number of nucleating agents, including the in-situ preparation of nucleating from aluminum isopropoxide and p-tertiary-butylbenzoic acid. Binsbergen also showed that nucleation could be achieved through addition of organic acids directly to PP when the PP contained at least 5 ppm of Al (see p. 264 in his Part I). However, to achieve good nucleation by this route required excess acid, and the latter is generally an unwanted source of volatiles, odor, corrosion, etc. Further, Rybnikar in his article, "Character of Crystallization Nuclei in Isotactic Polypropylene", Journal of Appled Polymer Science, Vol. 27, 1479-1486 (1982) showed that metastable nuclei, representing the unmelted crystalline phase of PP, stabilized by solid heterogeneities, can survive relatively high melt temperatures for practical melt processing times within limits of practical melt processing times.
While the above references have suggested various routes to nucleating polypropylene, there still remains a number of areas where improvement is needed. For example, nucleation without pelletization is desired for those new PP products now emerging. In addition, improved dispersion of the nucleant is also desired. Further, there are problems associated with nucleating polypropylene made from the new super high activity (MgCl.sub.2 -supported) catalysts.